Process automation field devices may be discrete devices used in a process automation system for the measurement of some physical phenomena. In a conventional field device, the field device may include electronics such as a microcontroller, memory, transducers, and analog-to-digital converters. The field device may include digital and analog outputs, for example, a 4-20 mA current output or a 1-5 V voltage output.
The field device may include wiring terminals by which the field device is connected to the process automation system. The field device may include terminals for power input and additionally include terminals for connection to the field device's 4-20 mA current output or 1-5 V voltage output. Because the field device's terminals are often screw-type terminals to which a stripped wire is connected, it is possible—and not always difficult—to incorrectly connect to the field device to the process automation system. For example, a current meter or a voltmeter to be connected to the field device's output terminals may instead be mistakenly connected to the field device's power input terminals, and a power supply to be connected to the field device's power input terminals may instead be mistakenly connected to the field device's analog output terminals.
In a field device that includes a 1-5 V analog output, the analog output circuit may include an operational amplifier (“op-amp”) to produce the 1-5 V analog output voltage. Op-amps typically have a low internal resistance at the output, and mistakenly connecting a power supply to the output terminals of the field device may draw an excessive amount of current into the output of the op-amp and thus burn the op-amp. Such a burned op-amp and output circuit would render the field device essentially unusable.
Various methods have been employed to protect the op-amp from excessive current draw when a voltage is mistakenly connected to the op-amp's output. One method includes placing a fuse in the output circuit that will blow if excessive current passes through the output circuit. However, a blown fuse requires replacement before the field device is again usable, and the fuse replacement generally requires a maintenance technician to access the field device.
Another method to protect the op-amp includes placing a current-limiting resistor in the output circuit. However, the current-limiting resistor may require a re-tuning of any feed-back loop employed with the op-amp. Further, the current-limiting resistor may affect the output voltage when the current-limiting resistor's resistance is comparable to the resistance of the process automation system. For example, if a 4.7 kΩ current-limiting resistor were placed in the output circuit, and if the process automation system's voltmeter had an internal resistance of 1 MΩ, in the voltage divider formed by the current-limiting resistor and the voltmeter, the 4.7 kΩ resistance is negligible:1 MΩ/(4.7 kΩ+1 MΩ)=0.995.  (Eq. 1)That is, the voltmeter is able to measure 99.5 percent of the 1-5V output.
However, it is not uncommon that the process automation system may use a voltmeter having an internal resistance of only 100 kΩ. In this case, in the voltage divider formed by the current limiting resistor and the voltmeter, the 4.7 kΩ resistance is not negligible:100 KΩ/(4.7 kΩ+100 kΩ)=0.955  (Eq. 2)That is, the voltmeter is able to measure only 95.5% of the 1-5V output. In this example, the 4.7 kΩ resistance has a larger effect and thus may limit the operation of the voltage output circuit. For this reason the current limiting resistor has limited application in the protection of the output circuit.
Accordingly, there remains a need for further contributions in this area of technology.